1. Field of the Invention
This invention relates to a method of minimizing the thermal decomposition of N-methyl-2-pyrrolidone, hereinafter referred to as NMP for the sake of brevity. More particularly, this invention relates to a method of minimizing the thermal decomposition of NMP by the addition of minor amounts of water thereto prior to its being heated to high temperatures. Still more particularly, this invention relates to a method of stabilizing the NMP solvent present in lube oil raffinate and extract phases wherein the NMP is separated therefrom via thermal solvent recovery means, said stabilization being accomplished by injecting small amounts of water into said solvent-containing phase prior to introducing same into a preheating furnace for subsequent distillaton or flash evaporation.
2. Description of the Prior Art
NMP has been known as a solvent for the separation of olefins, for the recovery of acetone from petroleum gas, for the extraction of naphthalenic hydrocarbons from various hydrocarbon mixtures, as a chemical reaction medium, polymer solvent, for use in industrial cleaning, for decolorizing petroleum oils and waxes, in paint removers, as a deicer for jet fuels and gasoline, etc. Perhaps the most important and widest industrial use of NMP is as an aromatics extraction solvent in various petroleum refining processes. Illustrative processes are for the separation of benzene, toluene and xylene or the well known BTX process, the recovery and separation of relatively pure, single-ring aromatics such as xylene, benzene and toluene from relatively light hydrocarbon mixtures known in the industry as the Arosolvan process, and in the extraction of aromatics from lube oil fractions in order to produce lube oils of relatively high VI and UV stability. Within the past ten years, a considerable amount of industrial research and development has been expended by the petroleum industry towards the utilization of NMP in lube oil deasphalting, extraction and dewaxing.
Concomitant with increasing interest in the use of NMP for solvent extraction processes, considerable effort has also been directed toward solvent recovery, particularly where NMP is used for solvent refining lube oil fractions and wherein the NMP must be removed from these higher boiling fractions by thermal or extractive means. The most conventional means for removing and recovering extraction solvents from lube oil stocks are flash evaporation and/or distillation, usually followed by steam or inert gas stripping. These recovery methods are the most widely used for extraction solvents such as phenol and have also been suggested for recovering NMP from extracted lube oil fractions.
Solvent extracting lube oil fractions using NMP produces raffinate and extract phases containing NMP. Because lube oil extraction with NMP is normally performed at temperatures below about 300.degree. F., it is necessary to heat the NMP containing phases to higher temperatures in order to separate the solvent therefrom if thermal recovery means are used. Generally, this temperature must be at least 400.degree. F. in order to separate the oil and solvent using either flash evaporation and/or distillation, because NMP boils at about 395.degree. F. or higher, depending on its purity. This minimum thermal separation temperature is readily achieved by heating the solvent containing phases in heat exchangers, such as direct fired tube furnaces, and then passing the hot raffinate and/or extract to flash towers, distillation towers, or combinations thereof, etc. The bulk temperature of the hot, solvent containing oil often exceeds 500.degree. F. and portions thereof may even exceed temperatures in excess of 700.degree. F. Material contained in the thin boundary layer film inside the furnace tubes may be heated to temperatures of 800.degree. F. or more, particularly in the radiant section of the furnace.
It has been found that NMP significantly decomposes when heated to temperature at or above about 700.degree. F. Therefore, it would be a significant improvement to the art if one could find a relatively simple and efficient way of minimizing the thermal decomposition of NMP, particularly when recovering same from extracted lube oil fractions employing thermal recovery means.